The significance of accepting runoff processes as nonlinear has been gaining considerable in recent times. However, it is hard to explore the types of nonlinearity acting underlying the runoff processes and the intensity of the nonlinearity at different timescales. Daily runoff time series observed at the Pingshan hydrometric station are used for this study. An attempt is made to identify the existence of chaos and the intensity of nonlinear behavior at three characteristic time scales (one day, 1/3 month, and one month). Six nonlinear dynamic methods are used: (1) phase space reconstruction and the delay time is estimated using average mutual information; (2) the sufficient embedding dimension is estimated using the false nearest neighbor algorithm; (3) correlation dimension method; (4) Lyapunov exponent method; (5) 0–1 test algorithm for chaos; and (6) the multi-step Volterra adaptive method. A comparison of results reveals the presence of low-dimensional chaos in the runoff dynamics at the various time scales and the time scales composes only a limit fraction of the intensity of nonlinear behavior. The reasonably good predictions indicate the efficiency of the nonlinear prediction method for predicting the runoff series.

Reservoir operation optimization is a technical measure for flood control and is beneficial owing to its reasonable and reliable control and application of existing water conservancy and hydropower hubs, while ensuring dam safety and flood control, as well as the normal operation of power supply and water supply. Considering the beneficial functions of reservoirs, namely flood control and ecological protection, this paper firstly established a two-objective optimal operation model for the reservoir group in the middle reaches of the Yellow River. We aim to maximize the average output of the cascade reservoir group and minimize the average change in ecological flow during the operation period under efficient sediment transport conditions, with the coordination degree of water and sediment as the constraints of reservoir discharge flows. The paper aims to construct an evaluation index system for reservoir operation schemes, apply a combined approach of objective and subjective evaluations, and introduce the gray target and cumulative prospect theories. By uniformly quantifying the established scheme evaluation index system, screening the reservoir operation schemes with the fuzzy evaluation method, and selecting the recommended scheme for each typical year, this paper provides a new scientific formulation of the operation schemes of reservoirs in the middle reaches of the Yellow River. The selected schemes are compared with actual data, demonstrating the effectiveness of joint reservoir operation and for multidimensional benefits in terms of power generation, ecology, and flood control.

Sediment problems affect the operation life and comprehensive benefits of a reservoir. In response to the floods in the middle and upper reaches of the Yellow River, the Xiaolangdi Reservoir is subjected to preventative pre-discharge scheduling. Through a prototypical observation of the density current plunging zone of the Xiaolangdi Reservoir in 2018, we collected prototypical data of the water level, water-sediment characteristics, sediment thickness, thalweg points, and flow velocity, and analyzed the pre-dam distribution of density current and scouring-sedimentation characteristics during the flood dispatching period of the Xiaolangdi Reservoir. The research results promoted the understanding of the movement law of the density current of the Xiaolangdi Reservoir and the development of efficient sediment drainage technology for reservoirs.